1. Field of the Invention
This invention relates to electrically conductive molecular complexes.
2. Background of the Invention
Conductive polymers (xcfx80-conjugated polymers) are potentially useful as a polymeric coating materials to impart special electrical, optical and electroactive properties to coated surfaces. When used to coat on metals it can impart protection against corrosion of the metals. See DE4334628 and U.S. Pat. No. 5,532,025. The electrically conductive form of the conducting polymers can also be coated on non-conductive surfaces to render the surface film to be electrically conductive. Examples of the xcfx80-conjugated polymers are polyaniline, polypyrrole, polyacetylene, polythiophene, etc.
The xcfx80-conjugated polymers are electrically conductive when it is doped by ionic compounds. In the electrically conductive state, the xcfx80-conjugated polymer backbone is a polycation. The positive charge on the ic-conjugated polymer backbone is the mobile charge that leads to electrical conductivity. The dopants are the counter ions that balance the positive charges. The difficulties in using conventional conducting polymers for coatings are associated with two of their properties; (1) they are unstable in their doped state and (2) they lack processability. The reason for the lack of processability comes from the fact that the conducting polymers are xcfx80-conjugated polymers. The delocalized xcfx80 electronic structure leads to a stiff polymer chain and strong inter-chain attraction. Thus, the conventional conducting polymers cannot be easily dissolved, melted or blended with other polymers.
The lack of material stability comes from the fact that the ionic dopants are easily lost or segregated from the conventional xcfx80-conjugated polymers. Examples of the dopants that have been used include hydrogen chloride, p-toluene sulfonic acid, 4-dodecylbenzene sulfonic acid, and dinonylnaphthalenedisulphonic acid (Jen et al., U.S. Pat. No. 5,069,820, Dec. 3, 1991; Elsenbaumer, U.S. Pat. No. 5,160,457, Nov. 3, 1992; Cao et al., U.S. Pat. No. 5,232,631, 1993; Kinlen U.S. Pat. No. 5,567,356, Oct. 22, 1996). When these conducting polymers are exposed to heat, water, solvents and/or moisture, these molecular dopants are lost. Once the dopants are lost, the polymer loses its electrical conductivity and its electroactivity. The loss of dopants occurs either during the manufacturing process or during the service life of the coated product. In certain cases, molecular anions with bulky organic groups were used to reduce the rate of loss of the dopant.
This only slows down the rate of dopant loss, it does not eliminate the problem. Even when the dopant is not lost from the coating, the electrical conductivity can be lost due to the diffusion of dopants at a microscopic length scale. The detachment of the dopants from the ic-conjugated polymer backbone at a microscopic length scale (0.1 pm length) leads to dedoping. A microscopic scale phase segregation between the polymer and the dopants are easily promoted by heat or solvent. The molecular dopants tend to segregate from the vicinity of the polymeric chain of the ic-conjugated polymer backbone which results in a loss of the desirable properties.
A problem with the conventional xcfx80-conjugated polymers is that they are brittle, hard and solid. In coating applications, the conventional xcfx80-conjugated polymers do not adhere to the surface of the substrate. Thus the xcfx80-conjugated polymers are blended with an insulating, non-conductive resin so that the mixture could be adherent to the surface of a substrate. See U.S. Pat. Nos. 5,532,025, 5,543,084 and 5,556,518. When the conducting polymer is imbedded in a matrix of a non-conducting polymer such as epoxy resin, polyurethane, polyacrylate or alkyd binders, the rate of dopant loss is reduced in the macroscopic level (e.g. 0.1 mm length), but the problem of segregation at a microscopic length scale (e.g. 0.1xcx9ctm length) is not eliminated. The electroactive properties will show signs of degradation over a period of several months. For a number of applications, the material stability is not good enough. In addition to the problem with the service life of coatings or blends of these xcfx80-conjugated polymers, there are problems with the manufacturing process. The dopants are easily lost during the manufacturing process either because of heat or because of contact with water or polar solvents. For example, U.S. Pat. No. 5543084 disclosed a method for electrocoating a blend of epoxy and polyaniline. The conductive polymer PANI-PTSA (polyaniline doped by p-toluenesulfonic acid) was mechanically blended in aqueous solution and then electrophoretically coated on metal. From the disclosure it is evident that the anionic dopant of PANI-PTSA was lost before the xcfx80-conjugated polymer was co-deposited with epoxy. A redoping by immersing the coating in camphor sulfonic acid was needed to restore polyaniline to its electrically conductive state. It is expected that the dopants incorporated by redoping will be easily dedoped again by either heat or by exposure to moisture.
Coatings that use undoped polyaniline (emeraldine base) have been disclosed in the literature (McAndrew et al. U.S. Pat. No. 5,441,772, and Epstein et al. U.S. Pat. No. 5,824,371). These xcfx80-conjugated polymers without dopant are nonconductive because there is no charge carrier on the polymer backbone. These non-conducting polymer coatings do not have the comparable performance as a coating material. For most applications it is essential to maintain the xcfx80-conjugated polymers in the electrically conductive state. Thus it is desirable to have an electrically conductive polymer that is both processable and is stable against the loss of dopants.
An alternative to the above mentioned remedies is to synthesize a molecular complex of the xcfx80-conjugated polymer and a polymeric dopant. If the polymeric dopant is strongly bonded to the xcfx80-conjugated polymer the dopant will not be easily lost during the manufacturing process and the service life of the conducting polymer. A method was previously disclosed for synthesizing processable conducting polymers with stable dopants (Liu et al. U.S. Pat. No. 5,489,400). In this disclosure, a template-guided chemical polymerization was used to obtain a polymeric complex that contained a strand of polyaniline and a strand of a polyelectrolyte. The reaction product is a non-covalently bonded molecular complex between a conducting polymer and a polyelectrolyte. The molecular complex contains the two linear chains of the component polymers bonded in a side-by-side fashion. The complex is a double-strand synthetic polymer. When polyaniline is the conductive strand, dsPAN designates the double-strand polyaniline. Compared with the double-strand biopolymer, DNA, the synthetic dsPAN is less ordered in structure and is generally not in a helical conformation. Examples of the polyelectrolytes are poly(styrenesulfonic acid) and poly(acrylic acid). Since the two strands of polymers are bonded strongly, these polymeric complexes are stable and do not dedope easily.
The dsPAN disclosed in this ""400 patent is one of three types. The first type is a water-soluble polymeric complex of polyaniline. This type of dsPAN is not suitable for anticorrosion coating applications because a pure dsPAN coating is redissolved in contact with water therefore the coating is lost in rain or humid air. It is conceivable that the water-soluble dsPAN be incorporated in a polymeric binder that prevents water dissolution of the coating. The hydrophilicity of this type of dsPAN is, however, still a problem for corrosion protection. The coating will absorb moister or swell in water thus reduce the adhesion of binder to the metal substrate.
A second type of dsPAN disclosed was an insoluble solid that precipitates from the aqueous reaction medium. This type of dsPAN can only be mixed with the binder by vigorous mechanical mixing (in a manner similar to that used for blending single-strand PANI-PTSA with epoxy described in Example 13 of Kinlen et al. U.S. Pat. No. 5,543,084). Although a blend made in this manner overcomes the problem of dedoping in U.S. Pat. No. 5,543,084, it is still not ideal. The dispersion contains large and brittle particles. The particles are not small enough for optimal polymer-metal interaction even when the precipitated particles are ground with a ball mill. The large particles do not xe2x80x98wetxe2x80x99 the metal surface. Another problem is that the mechanically stirred suspension is not a stable dispersion. It is difficult to maintain a uniform and stable suspension for large scale industrial production.
The third type of dsPAN disclosed in the ""400 patent is a colloidal suspension of small particles. Although the particle size is suitable for the electroactive polymer to interact with the metal surface to impart protection of the metal, the concentration of the colloidal particles in water is quite low (less than 1 g m of colloidal particles per liter of water). This low concentration is incompatible to the preferred high-solid content coating formulation.
PCT Publication WO 97/03127 discloses a chemically modified dsPAN that is soluble in polar organic solvents and can be applied to metal surfaces as a paint. The coating disclosed protecting metals from corrosion. These organic soluble dsPANs overcame the water absorption (swelling) problem of the water-soluble dsPAN disclosed in Liu et al. U.S. Pat. No. 5,489,400. This type of dsPAN is suitable for use as either a solvent-based paint or as a blend with hydrophobic epoxy oligomer for a thermoset coating. This type of dsPAN does not swell in water so it is suitable to be used as a coating. Although the coating is suitable for certain applications, a thin film of this material lacks mechanical strength.
The composition of the invention contains double strand conducting polymer (e.g., double-strand polyaniline and double-strand polypyrrole) with chemical functional groups in the second strand (the polymeric dopant) of the polymeric complex. The functional groups provide sites for chemical reactions after the coating or thin film formation step. One example of using the composition is to form a mechanically stronger film by a two-step process. In the first step of the process, the conductive polymer composition is either coated on a substrate or extruded to form a sheet or a fiber. In the second step of the process, the chemical reaction of the functionalized polymers is initiated to cause cross-linking. The cross-linked film or fiber has improved mechanical strength because in addition to the weaker van der Waals force for physical aggregation, there are covalent chemical bonds to maintain the integrity of the film or the fiber. The material is thus an improvement over the material disclosed in PCT Publication WO 97/03127 because the cross-linking provides mechanical strength to the material. For freestanding films or fibers, the mechanical strength of the cross-linked conducting polymer is better than the prior art materials.
The present invention maintains the advantage of a water-borne coating or film forming process, and at the same time avoids the swelling problem by allowing a cross-linking process following the water-borne coating process. In addition to the advantage of eliminating swelling, the cross-linked conducting polymer provides additional advantages. Compared to the non-cross-linked polymers, the cross-linked polymer forms coating with better hardness while retaining toughness. The coating also provide good adhesion to substrates.
The advantageous functionalization is easily obtained with the double-strand functionalized conducting polymer. It is difficult to achieve the same advantage in single-strand conducting polymer. The functional groups of the present invention resides in the second strand of the polymeric complex. The advantage is that there are a large number of possibilities of attaching functional groups on the second strand without impeding the electronic, optical and the electroactive properties of the first strand (the conducting polymer). In a single-strand conducting polymer with small dopants, the functional groups need to be attached to the conducting polymer. It is known in the literature that the additional functional groups degrade the electronic, optical and electroactive properties of the conducting polymer.
The present invention comprises a composition that retains the advantageously water-borne property to those disclosed in the publications, xe2x80x9cConducting Polymers for Coatings and Antielectrostatic Applicationsxe2x80x9d Sze C. Yang, Huaibing Liu, Robert Clark, U.S. Patent filed Oct. 29, 1997, PCT/U.S. 98/23032, filed Oct. 29, 1998 and xe2x80x9cWater-borne Anticorrosive Coating Composition Comprising a Polymeric Complex of Polyaniline and Polymeric Ionsxe2x80x9d Sze C. Yang and Richard Brown, PCT/US 99/28307, filed Dec. 1, 1999), but at the same time avoids the problem of water or solvent induced swelling that the compositions disclosed in the publications exhibit. In addition, the new composition enhances mechanical strength (cohesive strength) and adhesion to substrate (adhesive strength).
Broadly, the invention comprises a family of two-component polymeric complexes of xcfx80-conjugated polymers that are suitable for water-borne coating applications. One of the component (the first strand) contributes to the electronic, optical and electroactive properties. The other component (the second strand) contributes to the functionalizations that are advantageous for various applications. The invention also embodies the process of making the polymeric complexes, the use of the polymeric complexes in (1) anti-corrosion, (2) anti-electrostatic, (3) electrochromic formulations, a family of coating compositions containing the polymeric complexes, the method of applying the coating composition on a surface and the coating compositions per se.
More particularly, the present invention comprises a water-borne coating composition comprising a polymeric complex between a xcfx80-conjugated polymer, a polymeric ion (that serves as a dopant for the xcfx80-conjugated polymer), and a non-conductive polymer with functional groups. The functional groups allow for reactions that improve the properties of the coating.
More particularly, the invention comprises a coating that is a water-borne coating that can be cross-linked by reactions of the functional groups of the second strand and one or more components in the composition.